Mass spectrometric analysis of a sample requires that the sample be provided in the form of a gas or molecular vapor and then ionized. Ionization may be performed in the mass analyzing portion of a mass spectrometer, i.e., in the same low-pressure region where mass sorting is carried out. Alternatively, ionization may be performed in an ion source (or ionization device) that is external to the low-pressure regions of the mass spectrometer. The resulting sample ions are then transmitted from the external ion source into the low-pressure mass analyzer of the mass spectrometer for further processing. The sample may, for example, be the output of a gas chromatographic (GC) column, or may originate from another source in which the sample is not initially gaseous and instead must be vaporized by appropriate heating means. The ion source may be configured to effect ionization by one or more techniques. One class of ion sources is gas-phase ion sources, which include electron impact or electron ionization (EI) sources and chemical ionization (CI) sources. In EI, a beam of energetic electrons is formed by emission from a suitable filament and accelerated by a voltage potential (typically 70 V) into the ion source to bombard the sample molecules. In CI, a reagent gas such as methane is admitted into the ion source conventionally at a high pressure (e.g., 1-5 Torr) and ionized by a beam of energetic electrons. The sample is then ionized by collisions between the resulting reagent ions and the sample. The resulting sample ions may then be removed from the ion source in the flow of the reagent gas and focused by one or more ion lenses into the mass analyzer. The mass spectrometer may be configured to carry out EI and CI interchangeably, i.e., switched between EI and CI modes according to the needs of the user.
High-pressure CI ion sources have been employed in conjunction with three-dimensional (3D) quadrupole ion trap mass spectrometers, and would also be applicable to two-dimensional (2D, or “linear”) ion trap mass spectrometers (linear ion traps, or LITs). With either 3D ion traps or LITs, the sample is often introduced into the external ion source at an elevated temperature, such as when the sample is the output of a GC column. When the sample is provided at an elevated temperature, it is necessary to heat the ion source to prevent the sample from condensing in the ion source. However, because the ion source in this case is external to the ion trap and the ion trap itself is not utilized for ionization, it is not necessary to also heat the ion trap in this case, which is an advantage of external ion sources. Yet conventional external CI ion sources operate at high pressure as noted above, which is a disadvantage. High pressure CI requires the use of compressed gas cylinders to supply the reagent gas, as well as vacuum pumping stages between the ion source and the very low pressure ion trap. High pressure CI may increase contamination of the ion source, particularly in the area around the filament utilized to emit electrons where the high temperature causes pyrolysis of the reagent gas and contamination. High pressure also limits the choice of reagent gases able to be utilized and thus also limits the choice of chemical properties and reaction pathways available for CI. High pressure also limits the CI yield. Because ions are not trapped in a high-pressure ion source, the time in which the sample can interact and react with the reagent ions is limited by the volume of the ion source and the total gas flow rate. The gas flow rate in a high-pressure ion source is high, and thus the residence time of sample molecules in the ionization region where the reagent ions reside is low.
As an alternative to external ion sources, a 3D ion trap itself may be utilized to effect CI. In this case, the reagent ions are formed directly in the interior region defined by the electrodes of the 3D ion trap and the sample is subsequently introduced into the same interior region. In this case, the sample is ionized in this interior region and the resulting sample ions are subsequently scanned from the same interior region to produce a mass spectrum. Internal ionization is advantageous because it is performed at the low operating pressure of the ion trap. However internal ionization is disadvantageous because, unlike external ionization, it is necessary to heat the entire electrode assembly of the ion trap to prevent the sample from the GC from condensing on the electrodes. Operating the mass analyzer at elevated temperatures is disadvantageous in that it requires heating equipment and may produce inaccurate spectral data due to sample adsorption on the large surface area of the electrodes. Moreover, the electrode assembly must be fabricated by special techniques designed to enable the electrode assembly to reliably withstand repeated high-temperature operation.
In view of the foregoing, there is a need for providing apparatus and methods for implementing low-pressure EI and CI in which the sample is ionized in an ion processing device that is external to an ion trap utilized for mass analysis.